1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal orientation film. Besides, the present invention relates to a liquid crystal orientation film obtained by the method for manufacturing concerned and an optical film using at least one liquid crystal orientation film concerned. A liquid crystal orientation film of the present invention may be used as optical films, such as retardation plate, viewing angle compensation film, optical compensation film, and elliptically polarized light film, independently or in combination with other films. Furthermore, the present invention relates to visual displays, such as liquid crystal display, organic EL display, PDP, etc. using the above described optical films.
2. Description of the Prior Art
As methods of producing a liquid crystal orientation film, a method is known until now in which liquid crystal polymer is oriented at a liquid crystal temperature range and then cooled below a glass transition temperature. Since a raw material is a liquid crystal polymer, the method for manufacturing a liquid crystal orientation film has a characteristic that high film-forming property in a state where it is processed into a coating liquid is demonstrated, and therefore satisfactory workability is assured. However, generally, orientation of liquid crystal polymers is difficult as compared with low molecular liquid crystal compounds, and durability after oriented also has large dependence on a glass transition temperature of the liquid crystal polymer, and therefore, in general, it is difficult for high durability and high orientation property to be coexistent in a liquid crystal orientation film obtained by a liquid crystal polymer.
On the other hand, as a method for producing a liquid crystal orientation film, a method is known in which a polymerizable liquid crystal monomer is polymerized and cross-linked by ultraviolet radiation etc. after it is oriented at a liquid crystal temperature range. Since a polymerizable liquid crystal monomer is a low molecular compound, it shows comparatively satisfactory orientation property. Besides, when a liquid crystal monomer with polyfunctional reaction groups therein etc. in polymerizable liquid crystal monomers is used, a feature will be realized that a film formed after polymerization and cross-linking shows a high heat resistance and high strength. However, even if film formation is tried using a liquid crystal monomer as a coating liquid, uniform film is difficult to be formed and there occurs a problem that outward appearance-faults, such as crystal deposition is easily given. Besides, problems of inferior workability that a low viscosity of coating liquid deteriorates handling may occur.
In order to uniformly orient the above described liquid crystal polymer or the polymerizable liquid crystal monomer, liquid crystal molecule is preferably oriented on an orientation substrate. As orientation substrate, a substrate on which formed an oriented thin film, such as polyvinyl alcohol and polyimide that rubbed in one direction by rayon cloth etc., and stretched orientation films, such as polyester films and polyvinyl alcohol films etc. are known. Besides, optical orientation films, such as polyvinyl cinnamates, are also known.
However, the above described orientation substrates are unnecessary in optical characteristics. Therefore, when a liquid crystal orientation film is used as an optical film of a visual display, an orientation substrate is separated and removed from a liquid crystal orientation film, in consideration of defects, such as problem of thickness, and adverse influence of an orientation substrate itself on optical characteristics. When a stretched orientation film is used as an orientation substrate, adverse influence on optical characteristics is especially greatly demonstrated.
As liquid crystal polymers used for production of a liquid crystal orientation film, since it gives easy orientation, generally a polymer having approximately 5,000 through 50,000 of weight average molecular weight is usually used. However, in many cases, a polymer having a molecular weight in this range does not give a liquid crystal orientation film with self-supporting property. Therefore, inferior workability is given when an orientation substrate is separated from the liquid crystal orientation film obtained from the liquid crystal polymer.